The Study Of Wolf-rayet Stars: Formed Through Chemically Homogeneous Evolution

Massive stars are stars which showing O-type or early B-type spectrums（later evolve into the corresponding late-type spectrums）,and death as Supernova explosion（SNe）,which indi-cating the initial mass should be generally larger than 8 M_⊙.They supply their galaxies with nuclear reaction products continuously via their stellar winds or ultimate death as core-collapse supernova,they are also considered as the prime sources for Interstellar Medium（ISM）.The rotation rate,considered as one of the main initial stellar parameters,has many effective effects on stellar rotation,especially when the rotational velocity is too high(～400kms^-1)that makes the star more brighter and contract,and then evolve-up and bluewards in the HRD.This type of evolution is referred as”quasi-chemically homogeneous evolution”.Wolf-Rayet star（WRs）is the advanced evolutionary stage of massive star with wind den-sity usually an order of magnitude higher than massive O stars,the high stellar wind mass loss rate makes WRs dominating the feedback to their galaxies.At the same time,having a knowl-edge of the formation channels of WRs can provide the criteria to judge the various physical processes of massive star evolution（such as massive rotation and material loss）of the theoretical model.Chemically homogeneous evolution has been proposed as a pathway for the formation of WRs.In this paper,using the stellar evolution code—Modules for Experiments in Stellar Astro-physics（MESA）,we investigate the evolution of massive stars with different rotational veloci-ties and metallicities,the initial rotating velocities are taken as 0,250,500 and 650 kms^-1,and the metallicities Z equal to 0.02 and 0.014 for Milky Way,0.008 and 0.006 for LMC,0.004 and0.002 for SMC and Z=0.001 for galaxy with low metallicity.And we analysis our results with observation in the HRD.We find that the rotation has a few effects on the evolution of massive stars with high metallicity but rapidly rotating massive stars with low metallicity can easily evolve into WR stars due to the rotation results in chemically homogenous evolution.This can explain the origin of single WR stars in galaxy with low metallicity.At the same time,due to rotation leading to chemically homogeneous evolution for massive stars with low metallicity,the masses of non-hydrogen elements（He,C,N,O Ne,Mg and Si）losted by these stars with high rotational velocity are 10～1000 times higher than those for non-rotation stars.